Biodiesel is a fuel derived from biologically sourced fatty acids such as fatty acid glycerides or fatty acid esters from lipid containing plant material, microbes, or animals which can be used to replace petroleum derived fuels. Typically, it is blended with petroleum fuels in blends from less than 1 wt. %, known as B1, to pure biodiesel, known as B100.
Commonly, biodiesel is a mono-alkyl ester derived from the processing of vegetable oils and alcohols. The processing is typically carried out by an esterification reaction mechanism, and typically is performed in an excess of alcohol to maximize conversion. Esterification can refer to direct esterification, such as between a free fatty acid and an alcohol, as well as transesterification, such as between an ester and an alcohol. While vegetable oil and alcohols are commonly employed as reactants in esterification reactions, a fatty acid source such as free fatty acids, soaps, esters, glycerides (mono-, di- tri-), phospholipids, lysophospholipids, or amides and a monohydric alcohol source, such as an alcohol or an ester, can be esterified. In addition, various combinations of these reagents can be employed in an esterification reaction.
Vegetable oils include triglycerides and neutral fats, such as triacylglycerides, the main energy storage form of fat in animals and plants. These typically have the chemical structure C3H5(OOCRx)3 where Rx represents a saturated or non-saturated hydrocarbon chain. Different vegetable oils have different fatty acid profiles, with the same or different fatty acids occurring on a single glycerol. For example, an oil can have linoleic, oleic, and stearic acids attached to the same glycerol, with Rx representing all three of these fatty acids. In another example, there can be two oleic acids and one stearic acid attached to the same glycerol, with Rx representing all of these fatty acids. A triglyceride consists of three fatty acids (e.g., saturated fatty acids of general structure of CH3(CH2)nCOOH, wherein n is typically an integer of from 4 to 28 or higher) attached to a glycerol (C3H5(OH)3) backbone by ester linkages. In the esterification process, vegetable oils and short chain alcohols are reacted to form mono-alkyl esters of the fatty acid and glycerol (also referred to as glycerin). When the alcohol used is methanol (CH3OH), a methyl ester is created with the general form CH3(CH2)nCOOCH3 for saturated fatty acids. Typically, but not always, the length of the carbon backbone chain is from 12 to 24 carbon atoms.
The esterification process can be catalyzed or non-catalyzed. Catalyzed processes are categorized into chemical and enzyme based processes. Chemical catalytic methods can employ acid and/or base catalyst mechanisms. The catalysts can be homogeneous and/or heterogeneous catalysts. Homogeneous catalysts are typically liquid phase mixtures, whereas heterogeneous catalysts are solid phase catalysts mixed with the liquid phase reactants, oils and alcohols.
Homogeneous catalysts frequently yield the most effective reactions and fastest reaction rates. The primary disadvantage of the homogeneous catalysts is that downstream processes can be more complex because they must support the isolation and purification of the product to remove the homogeneous catalyst. Two approaches to removing homogeneous catalyst include water washing (mixing with water and separation of the water, or countercurrent contacting with water such as in a column) and resin (or, ion exchange) based purification. Water washing is effective but results in waste water disposal issues and can influence the oxidative resistance of the product biodiesel. The resin based purification methods can also be effective, but the resins are relatively costly and can be costly to regenerate or can result in solid waste disposal issues. Fresh catalyst is generally necessary because water washing and resin based purification do not allow the possibility of recycling catalyst, which increases feedstock costs. The reaction rates of homogeneous catalyst process can be enhanced by increasing the reaction temperatures and pressures. The reaction rates with homogeneous catalysts can also be increased by increasing the intensity of the mixing of the reactants in high sheer reactors.
Heterogeneous catalysts typically have slower reaction rates than homogeneous catalysts because the reactants must diffuse to the catalytic site prior to reacting. They are also subject to poisoning by impurities in the process feeds. The advantage of heterogeneous catalysts is that separation of the catalyst from the reaction products can be simpler, and disposal and purification issues in downstream processes are reduced. Typically, the heterogeneous catalyst is designed to be easily separated by a physical mechanism such as a filter, so that the separated catalyst can be recycled and reused. The heterogeneous catalyst can be fixed in the reactor as a flow through reactor. Performance of recycled catalyst can decrease with time and eventually needs to be replaced. As an alternative to adding heterogeneous catalyst in particle form to the reaction mixture, the surface of the reaction vessel can be treated to have catalytic activity and function as a heterogeneous catalyst. The reaction rates of heterogeneous catalysts can be enhanced by increasing the reaction temperatures, pressures, and mixing.
Non-catalyzed reaction mechanisms use process conditions such as elevated temperatures, elevated pressures and high-sheer mixing to increase the reaction rates and conversion efficiencies. These general approaches are known in the art of chemical processing in the chemical, petroleum and pharmaceutical industries. Generally, as the temperature is increased, the liquid phase reactants change state to a vapor or gaseous stage, which typically decreases the desired homogenous nature of the reaction mixture, and therefore, decreases reaction rates. Because of this, the reaction pressure is typically increased to decrease the vapor formation. However, if the increased temperature leads to vaporization of one or more reagents, the resulting removal of reagent from the reaction mixture can lead to a decrease in the yield and/or reaction rate. Increasing the system pressure can prevent this vaporization and resulting decreases in yield and rate. Typically, the pressure of the reaction is maintained above the vapor pressure of the reagents at the operating temperatures. The temperature and pressure can be taken to a condition known as the critical point, which is the point at which the liquid-vapor phase transition does not exist above the supercritical temperature and pressure. At this point the distinction between liquid and vapor ceases. The critical point for methanol is 512.6K and 79.8 atm and for ethanol is 513.9K and 60.6 atm. The reaction vessels and support equipment necessary to achieve these elevated temperature and pressure conditions can be expensive. While operating at extreme conditions can be desirable in certain respects, when the operating temperature of the reaction or the concentration of catalyst is increased too much, undesirable side reactions or undesirable side-products can result.